What is an IB (Infinite Baffle) Subwoofer System?

[collo]

What is an IB?

All speakers need to separate the front of the driver from the back of the driver in order to prevent an acoustic short circuit from cancelling the output.

A sealed box accomplishes this, but the air in the box acts as a spring, which changes the response and efficiency.

A subwoofer where the baffle is infinitely large can accomplish the required separation without the penalties that a sealed box imposes.

How large is "infinite"? If the sound has to travel a long way to complete the round circuit, the cancellation will be less. This will be the case where the back energy is radiated into a roofspace, or under the flooor of a house.

When modelling an IB, it is easiest to treat it as a huge sealed box.

If the box is 10 times the combined Vas of the drivers, the spring effect of the air is minimised to the point that it qualifies as an "Infinite Baffle" system. An adjacent room works well with this setup. If space is limited, this can be reduced to 4 * Vas.


Output

Because it is not working against an air spring, an IB will give extremely good transient response. Another result is that Xmax is reached quite easily. This is something that has to be considered by modelling the design in WinISD or Unibox if you're to avoid blown cones!

Many sub drivers now have an fs in the low 20's, which along with some room gain, means good bottom end response.

To get output down to the low teens, you need to move a lot of air, usually meaning four high excursion 15 or 18 inch drivers, or a larger number of less capable drivers.

The normal way to compare IB systems is by displacement, which is given by this formula:

displacement = number of drivers * Sd * Xmax * 2

For a small room you could get away with 10 litres, however for a large space, or for impressive output, you should go for
20litres or higher. You normally won't use more than 30litres.

For those who live outside the US, the local drivers may not fare so well. Don't dispair because some equalisation will help.

Behringer make a device called the feedback destroyer, which bands use to chase down and bury frequencies that cause feedback. The feature that is useful to the sub builder, is that these come with either 12 or 20 digital parametric equalisers at a very reasonable cost.

Combine this with the Room Equalizer Wizard software (REW) and you have a solution that can boost the bottom end and tame troublesome room resonances at the same time. Of course, to get high levels of output at low frequencies without exceding Xmax, you will need to add more drivers.

The most popular amp by far is the Behringer EP2500, which is a cheap source of abundant power. There are other amps of course, with offerings from Crown and Buttkicker also being popular.


Mounting options

The simplest option is to just mount the driver through the wall / roof / floor. This has the disadvantage of vibration. As more drivers are added to the mix, this becomes a serious issue, with movement cancelling some of the output. Such a design is called a line array.

A better option is to use a manifold. This is simply a box with one end opening into the room, with the easiest and most popular variation being a cube. Mounting the drivers in opposing pairs will cancel out the vibration. Manifolds also hide the drivers.

It doesn't matter which way you face the drivers, in or out, as the opening is usually covered with some grille cloth. Some designers like to use pairs of one driver in each direction to equalise any non-linearities in the magnetic gap, but any gain here is small.

If you're using more than four drivers, try to have the sound from all drivers arrive at the same time to prevent cancellation issues. It is better to have two cubes, or a single wider manifold than to have a long "tunnel" type manifold.

The opening has to be large enough to prevent the manifold acting as a bandpass box. A cube works fine, which for four drivers, has an opening that is about one half of the combined radiating areas of the drivers. If you're building a 6 or 8 driver manifold, stick to this ratio and you'll be OK. Avoid a long slot design.

If you're planning to roof mount a lot of drivers, consider using multiple manifolds to distribute the weight.


If you have the room to build an IB then do so. Do so now. You won't regret it!